Imagery collected using certain arrays of infrared detectors (or sensors) suffers from non-uniformity noise due to a number of factors, including variations between detector elements in dark current (leakage current), offset (bias) and responsivity (gain). Other sources of non-uniformity noise include electronic readout non-uniformities and non-uniformities of low spatial frequency including non-uniformities derived from device optics (e.g. due to the decrease in radiation from the center to the edge of the field of view,) and non-uniformities stemming from uneven cooling of detector elements. In many situations, the non-uniformity noise adversely affects image quality.
A number of methods have been tried for effecting a non-uniformity correction. Some methods use a uniform calibration source or reference image to compute pixel-dependent offset coefficients for each pixel. Alternatively, the image is defocused on the array of detector elements, and this is used as a reference image. Nevertheless, it is recognized that the calibration is good only at background temperatures near the reference temperature. In order to effect a non-uniformity correction valid over a wider temperature range, two point correction (TPC) techniques employing reference images taken against a uniform background at two different temperatures are commonly used.
According to the TPC techniques, reference coefficients derived from the reference images are stored in gain and offset tables, and the non-uniform correction is subsequently carried out in real-time. The reference images may be the average over a number of frames to suppress temporal noise. Although the gain coefficients are generally stable, the offset coefficients drift and can require frequent calibration. Towards this end, a reference or defocus block may be inserted every few minutes, or even in alternate frames using a chopper.
There is an ongoing need for improved techniques to compensate for non-uniformity noise in multi-element infrared detector arrays. Preferably, the technique would not employ image blurring, would not require use of complex mechanical systems, and would not require frequent recalibration.
Below is a list of US Patents and US published patent applications providing potentially relevant background art. Each of these US Patents and US published patent applications are incorporated herein by reference: U.S. Pat. Nos. 4,298,887; 4,795,904; 5,925,880; US 2002/0159101; US 2003/0198400.